Ultraviolet radiation produces different types of lesions on DNA which unrepaired could lead to lethal, mutagenic and tumorigenic effects. In the last decade there have been reports on the formation of purine photoproducts in UV irradiated DNA which could lead to deleterious effects. Photochemical studies on monomeric purines and their derivatives from our laboratory indicate photodestruction yields as high as the one observed for monomeric pyrimidines. Also, the formation of highly reactive intermediates have been observed. These could initiate reaction with other bases and/or molecules close to the DNA. Our main objectives are: 1. To study and characterize the excited states and reactive intermediates participating in the photodestruction and photochemical reactions of purine deoxynucleotide dimers, oligo and polynucleotides. These will be characterized from their absorption spectra and kinetic behavior obtained with laser and conventional flash photolysis and fluorescence spectrometry. These studies will provide information on the mechanism of formation of the recently observed purine photoproducts in DNA. 2. Identify the stable products in the photodestruction of deoxyguanosine and deoxyguanosine monophosphate and establish its photodestruction mechanism. 3. Identify the intermediates participating in purine photoinitiated cross linking reactions with organic compounds considered to be models for the side chain of amino acids. The information to be obtained will aid in the understanding of the detrimental effects of UV light on living systems such as mutations, aging and carcinogenesis.